Monday, December 28, 2009

Evidence from paleosols for the geological antiquity of rain forest

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Kaolinitic claystones in Paleozoic paleokarst underlying the Middle Pennsylvanian Fort Scott Limestone near Drake, Missouri, contain abundant fossil root traces. These include a surficial root mat as well as stout, woody, deeply penetrating root traces: a rooting pattern s similar to that under rain forest. Also similar to soils of rain forest is the deeply weathered clay of the paleosol, in which minimal amounts of nutrient bases remain. Forest communities adapted to oligotrophic clayey substrates in humid climates existed at least 305 million years ago.

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Fossil evidence of rain forest is sparse, because most rain forests colonize sites too well drained to favor the preservation of fossil plants and soils insufficiently calcareous to preserve bone and shell (1). Exceptional fossil biotas of well-drained wet forest as old as the Eocene are known from paleokarst (2). Fossil fruits, seeds, and leaves similar to those of Indo-Malesian rain forest also are known no further back in time than the Eocene (3). Fossil plants of Carboniferous coal measures have been interpreted as the remains of rain forest, but despite the discovery of a great diversity of Carboniferous plants including vines (4) and epiphytes (5), no Carboniferous plant assemblages are physiognomically comparable to Guineo-Congolian, Amazonian, or Indo-Malesian rain forest (6). Given this sparse fossil record and preservational biases, rain forests of the past may be better recognized from fossil soils than from fossil plants. Rain forests have high productivity and stature despite their oxidized oligotrophic clayey soils (7).

An ancient example of a deeply weathered and oxidized paleosol containing large woody root traces is in the Farnberg pit, 5 km southwest of Drake, Missouri (8). The pit is an excavation for refractory clay in the Cheltenham Formation. These clays and sandstones fill paleokarst with 46 m of relief into the Ordovician Jefferson City Dolomite (9). Elsewhere in this region this paleokarst is incised into Burlington Limestone, and so is younger than the Mississippian (9). The Farnberg clay paleosol is only 15 cm below the top of the Cheltenham Formation, which is overlain by the Fort Scott Limestone of Middle Pennsylvanian age [middle Desmoinesian (10)], correlative with the European Moscovian and Westphalian D some 305 million years old (11).

The Farnberg clay paleosol is overlain abruptly by gray clayey siltstone containing fossil logs, one of which is at least 179 cm long, 4 cm thick, and 28 cm wide. The width of compressed logs is thought to equal their original diameter (12). The clayey surface horizon includes a dense mat of root traces up to 3 mm wide that define a crude platy ped structure (Fig. 1B). Stout root traces up to 6 mm in diameter with the silky surface texture characteristic of wood were seen penetrating the paleosol to depths of up to 2 1 0 cm (Fig. 1, A and C).

Fossil root traces in the paleosol are surrounded by gray to green colored clay (Fig. 1, A and C). Such drab halos are commonly produced in paleosols a result of the anaerobic decomposition of remnant organic matter shortly after burial (13). This paleosol also may have been reddened by the dehydration of iron hydroxides to hematite during burial (13). Also likely is lithostatic compaction from overlying Paleozoic rocks 1.2 km thick according to estimates from regional isopachs and 1.5 km thick according to the regional variation in moisture of coals (14). Compatible with such a depth of burial is the high volatile bituminous rank of the Mulky Coal at the stratigraphic level of the Farnberg paleosol in Missouri (15) and the white to cream color of Pennsylvanian conodonts in this region (16). On the basis of standard compaction curves (17), the clayey paleosol is probably 76% of its former thickness. These alterations after burial do not alter the conclusion that large woody plants grew here in a thick clayey soil during Middle Pennsylvanian time.

Also little altered during burial was the chemical and mineral composition of the Farnberg paleosol, which is remarkably low in weatherable bases, represented by lime, magnesia, soda, and potash (Fig. 2 and Table 1). The analysis of clays of the Farnberg paleosol by x-ray diffraction revealed mostly well-ordered kaolinite. All of its characteristic peaks from 0[degree] to 30[degrees] 2[theta] were sharp, and there was little response to glycolation. Small amounts of submicroscopic hematite were also identified from x-ray peaks. No halloysite or diaspore could be identified by differential thermal analysis, but small amounts of dickite and metahalloysite could not be ruled out (Table 2). The absence of diaspore and illite is surprising, because diaspore is common in underlying claystones and illite is common in the capping clayey siltstone (9). In modern rain forest soils, kaolinite is maintained by biological activity against chemical equilibrium (18).


Some refractory clays in Missouri may have been created by hydrothermal fluids responsible for Pennsylvanian to Permian mineralization of Mississippi Valley type (19). Local high-temperature geothermal anomalies apparent from conodont (16) and coal maturation studies (15) or fluid inclusions of sphalerite mineralization (20) are all remote from the Farnberg pit, in southeastern Missouri and southern Illinois and Indiana. Even more distant is the likely source of mineralizing brines in overthickened Alleghenian sediments in Arkansas and Tennessee (21, 22). The Farnberg clay paleosol has much lower values of Co, Cr, Cul Li, Ni, and Zn than clayey alteration in the mineral districts (21), and low Ba/Sr ratios indicate only modest leaching (23). The eastern end of the Farnberg paleosol is both overlain and underlain by sandstone (Fig. 3), including rare unaltered amphibole, in a much broader and shallower paleokarst depression (9) than the cherty, sulfide-bearing, narrow, cylindrical, breccia pipes thought to have been hydrothermally altered (19, 21). Hydrothermal alteration is difficult to envisage from the distribution of the Farnberg paleosol sandwiched by illitic shale and diaspore clays and for correlative kaolinitic claystones sandwiched by illitic shaly sequences far to the north, east, and west (9, 10). This is not to say that the Farnberg paleosol has been unaffected by geothermal heating attendant on burial. The Mulky Coal of Missouri, which also underlies the Fort Scott Limestone, has a moisture-free calorific value of 6.4 kcal/g (12,795 Btu), a moisture of 10.5%, and a volatile matter of 39.5%, averaged from four analyses (15). On the basis of standard coalification curves (24), these values correspond to a maximum burial temperature of about 40[degrees]C. Regional low-color alteration of conodonts [conodont alteration index (CAI) of 1] indicates temperatures less than 50[degrees] to 80[degrees]C (16).

The Farnberg clay palcosol presents an example of an extremely oligotrophic clayey soil that supported forest and that is similar to modern soils of rain forest in Brazil, Zaire, and Malaysia (25). Such low-nutrient tropical soils are generally classified as Oxisols and Ultisols (26) or as Acrisols, Ferralsols, and Nitosols (27). We could not discern any significant subsurface clay enrichment like that of Acrisols, Nitosols, and Ultisols from point counting petrographic thin sections (Fig. 2) or clay mineralogical work (Table 2), although there are a few spectacular examples of clay skins (Fig. 1D). Also lacking are the spherical micropeds, high iron content, and abundant social insect burrows seen in many modern Ferralsols and Oxisols (28). The most similar modern soils are Acric Ferralsols (27) and Acrudox or Acroperox (26).


The deep weathering of bases and abundant kaolinite is characteristic of soils of very humid climates, with more than 2000 mm of mean annual rainfall (29). Ferruginous concretions (9) are evidence of climatic seasonality, but the Farnberg paleosol has no organized vertic structures of the sort found in soils of highly seasonal climates (21).

Vegetation of the Farnberg paleosol was mainly gymnosperm trees, on the basis of the stout, copiously branched, woody root traces. Nothing like the stout fleshy roots of tree lycopsids was seen, although a likely specimen has been reported in diaspore clay of the Cheltenham Formation only 16 km north of the Farnberg pit (9). Tree lycopsids, calamites, and ferns were widespread in Pennsylvanian lowland vegetation (30) and in the swampy bottoms of Early Pennsylvanian paleokarst in northeastern Illinois (31). The paleokarst fossil floras also include probable upland taxa seldom seen in coal measures, including the seed ferns Megalopteris and Lesleya (31). Broadleaf seed ferns are also found in association with Early Permian noncalcareous red paleosols in Texas (32) and Arizona (33). Conifers are another plant group of likely upland habitats, rare in lowland fossil plant assemblages of Middle Pennsylvanian age as fusinized fragments (34). This preservation may indicate that conifers preferred seasonally dry soils more prone to forest fire than is likely for the Farnberg paleosol.

Thick cross-bedded sandstone interbedded with the Farnberg paleosol (Fig. 3) is a paleochannel and indicates a connected drainage through this upper portion of the partly filled paleokarst. The Farnberg paleosol formed on well-drained valley slopes of this polje between hills of karst and chert of the Ozark highlands to the south and extensive seas and swamps of lowlands to the north (9). Parent material of the Farnberg paleosol included flint clays remaining from paleokarst dissolution, slope wash, and a small amount of fresh fluvial deposits and air-fall dust. Relict bedding can be discerned in the basal part of the profile, but none remains in the rooted horizons (Fig. 2). By comparison with modern soils (35), this observation indicates at least tens of thousands of years of soil formation. Soil formation could have extended for millions of years to create such deeply weathered material, but it is likely that a substantial component of this chemical weathering occurred in preexisting soils that were eroded provide the parent material of the Farnberg paleosol (9).

Fossil trees and forested paleosols are known to be as ancient as the Middle Devonian (22), but none of these Devonian paleosols are nearly as base-depleted as the Farnberg paleosol. Quartz sandstones with large fossil root traces represent other kinds of oligotrophic soils (Spodosols, Dystrochrepts, and Quartzipsamments) and, like the Farnberg paleosol, are also no older than the Carboniferous (22, 36). No Devonian or older orthoquartzites or bauxites are known to contain large woody root traces. These are difficult substrates for plants, and the coevolution of nutrient-conserving ecosystems capable of sustained growth on such soils may have taken some time. Whether it took as long as the Late Devonian and Mississippian for trees to adapt to such infertile soils remains to be seen from the continuing search for fossil root traces and paleosols.


(1.) G. J. Retallack, Paleobiology 59, 59 (1984). (2.) G. Krumbiegel, L. Ruffle, H. Haubold, Das Eozane Geiseltal (Ziemsen, Lutherstadt, Germany, 1983); M. Archer, S. J. Hand, H. Godthelp, Riversleigh (Reed, Sydney, 1991). (3.) J. A. Wolfe, Geophys. Mon. Am. Geophys. Union 12, 357 (1985). (4.) G. W. Rothwell, Paleontographica B151, 171 (1975). (5.) W. A. DiMichele and T. L. Phillips, Rev. Palaeobot Palynol 27, 103 (1979). (6.) V. A. Krassilov, Paleoecology of Terrestrial Plants (Wiley, New York, 1975). (7.) R. L. Sanford, Science 235, 1062 (1987). (8.) The Farnerg pit is 200 m east of the Bueker pit in the NE1/4, NW1/4. SW1/4 section 30 T43N R5W, Gasconade County. The Farnberg clay paleosol is in the upper level of the southern side of the pit and was measured and collected on 3 November 1989. (9.) W. D. Keller, J. F. Westcott, A. O. Bledsoe, in Clays and Clay Minerals, A. Swineford and N. Plummer, Eds. (National Academy of Sciences, Washington, DC, 1954), pp. 7-46. (10.) D. E. Zeller, Ed., Bull. Kans. Geol. Surv. 189 (1968); H. B. Willman et al., Bull. III. Geol Surv. 95, 1 (1975). (11.) W. B. Harland et al., A Geologic Time Scale 1989 (Cambridge Univ. Press, Cambridge, 1990). (12.) J. Walton, Philos. Trans. R. Soc. London Ser. B 226, 219 (1936). (13.) G. J. Retallack, Annu. Rev. Earth Planet. Sci. 19, 183 (1991). (14.) C. Beaumont, G. Quinlan, J. Hamilton, Tectonics 7, 389 (1988). (15.) H. H. Damberger, Econ. Geol. 66, 488 (1971); C. E. Robertson, Rep. Invest. Mo. Geol Surv. Water Res. 54, 1 (1973). (16.) A. G. Epstein, J. B. Epstein, C. D. Harris, U.S. Geol Surv. Prof. Pap. 995, 1 (1977); A. G. Harris, C. B. Rexroad, R. T. Lierman, R. A. Askin, Cour. Forsch. Inst. Senckenberg 118, 253 (1990). (17.) Compaction (C) as a fraction can be calculated from depth (D) in kilometers from

C = 0.5/{[0.49/[e.sup.(D/3.7])]-1} from J. G. Sclater and P. A. F. Christie [J. Geophys. Res. 85, 3711 (1980)]. (18.) Y. Lucas, F. J. Luizao, A. Chauvel, J. Rouiller, D. Nahon, Science 260, 521 (1993). (19.) H. A. Tourtelot, M. B. Goldhaber, M. R. Hudson, Geol Soc. Am. Abstr. Prog. 20, A140 (1988). (20.) D. L. Leach and E. L. Rowan, Geology 14, 931 (1986). (21.) A. V. Heyl, in International Conference on Mississippi Valley-Type Lead-Zinc Deposits, G. Kisvarsanyi, S. K. Grant, W. P. Pratt, J. W. Koenig, Eds. (Univ. of Missouri Press, Rolla, MO, 1983), pp. 27-60; C. Bethke, Econ. Geol 81, 233 (1986). (22.) J. C. Brannon, F. A. Podosek, R. K. McLimans, Nature 356, 509 (1992). (23.) G. J. Retallack, Soils of the Past (Unwin-Hyman, London, 1990). (24.) J. Karweil, in Petrographie Organique et Potential Petrolier, B. Alpern, Ed. (Centre National du Recherche Scientifique, Paris, 1975), pp. 195-203. (25.) P. A. Sanchez, Properties and Management of Soils in the Tropics (Wiley, New York, 1976). (26.) Soil Survey Staff, Keys to Soil Taxonomy (Tech. Monogr. 19, Soil Management Support Services, Blacksburg, VA, 1990). (27.) Food and Agriculture Organization, UNESCO Soil Map of the World (1971-1981). (28.) G. J. Retallack, Miocene Paleosols and Ape Habitats in Pakistan and Kenya (Oxford Univ. Press, New York, 1991). (29.) G. D. Sherman, in Problems of Clay and Laterite Genesis, A. F. Frederickson, Ed. (American Institute of Mining and Metallurgical Engineering, New York, 1952), pp. 154-161; C. E. Weaver, Clays, Muds and Shates (Elsevier, Amsterdam, 1989). (30.) W. A. DiMichele and R. W. Hook, in Terrestrial Ecosystems Through Time, A. K. Behrensmeyer, J. D. Damuth, W. A. DiMichele, R. Potts, H.-D. Sues, S. L. Wing, Eds. (Univ. of Chicago Press, Chicago, 1992), pp. 205-323. (31.) R. L. Leary, Rep. Invest. III. State Mus. 37, 1 (1981); Science 249, 1152 (1990). (32.) S. H. Mamay, Am. J. Bot. 76, 1299 (1989); G. J. Retallack, field observations, (33.) D. White, Carnegie Inst. Washington Publ. 405, (1929), p. 1; G. J. Retallack, field observations. (34.) A. C. Scott and W. G. Chaloner, Proc. R. Soc. London Ser. B 220, 163 (1983). (35.) P. W. Birkeland, Geomorphology 3, 207 (1990). (36.) S. D. Vanstone, J. Sediment. Petrol. 61, 445 (1991). (37.) Chemical analyses were done by C. McBirney using atomic adsorption, with errors from 10 replicate analyses of standard rock (W2). (38.) Thermal analysis by M. Foldvari and J. German-Heins.

31 January 1994; accepted 23 May 1994

G. J. Retallack, Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA.

J. Germin-Heins, Institute of Applied and Environmental Geology, Eotvos Lorand University, 1088 Budapest, Muzeum Korut 4/a, Hungary.

Source Citation
Retallack, Gregory J., and Judit German-Heins. "Evidence from paleosols for the geological antiquity of rain forest." Science 265.5171 (1994): 499+. Academic OneFile. Web. 28 Dec. 2009. .

Gale Document Number:A15650652

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